This card represents the subconscious mind. It represents intuition, deep understanding, wisdom, stability, patience, mystery and silence. It also shows inaction. It is the balancing force between pairs.
The pillars to her sides represent the opposites, light and dark, and she sits in the middle of them, impartial to one or the other. These columns represent all dualities, light and dark, good and evil, day and night, summer and winter. The curtain behind her connects the pairs (with veiled secrets behind it). The Priestess is holding a scroll of universal and personal knowledge, but only part of it is showing, because god has more yet to reveal. The High Priestess is the link, the conscious mind formulates ideas and the subconscious sets them into action in a fashion of order, represented by the block on which she sits.
This card falls under the vibration of the number 2. Whenever the card comes up in a reading, the high priestess warns the seeker to be watchful of his subconscious state. This card also advises him/her to look for the hidden path of wisdom.